I also wonder if he might have blow some of his inheritance on drugs, and that his funds are monitored by Mycroft to make sure he doesn't squander the rest of it away.

I did wonder if perhaps Mycroft was actually in control of Sherlock's money soon after his drug habit. I can see Mycroft doing that for a time until he felt like it was safe to let Sherlock control his own finances again. The time in between when he was essentially giving Sherlock an allowance was probably hellish for both parties.

I've often wondered if some of the animosity comes from Sherlock's drug use and Mycroft's response to it. Because if Mycroft has always been as protective as he is, it makes me skeptical that Sherlock could have been using without him knowing. And so I wonder if Mycroft's spying is in response to Sherlock's addiction, both worried about Sherlock and guilty that he didn't notice sooner.

That 'danger night' line always struck me as someone who had been there before, and dealt with the worst of it. I could see him not realizing how bad it was until it was very bad, and then just conquering the world to try to fix it. Mycroft is the best big brother ever while also being the worst.